This invention relates to an ionization chamber type X-ray detector adapted especially for use in a computerized X-ray tomography device.
For such a tomography device has been hitherto used an ionization chamber type detector which measures the spatial distribution of X-rays. The schematic structure of this detector is as shown in FIG. 1. Referring to the figure, alternate parallel flat anode and cathode electrodes 2 and 3 with a predetermined inverval defined therebetween are disposed between a pair of parallel electrode supporting plates 1 (made of, for example, insulating material). For practical use, this structure is placed in the atmosphere of heavy-atom gas (e.g. Xenon) kept at about 10-50 atmospheric pressures. X-ray coming on in the direction as shown by an arrow in FIG. 1, make interactions with the gas to produce photoelectron-ion pairs. Under the presence of an electric field, the photoelectrons are collected onto the anodes 2 while the ions are gathered by the cathodes 3. Accordingly, through a pair of anode and cathode electrodes flows a current proportional to the intensity of X-rays in the vicinity of these electrodes.
FIG. 2 shows a somewhat detailed structure of the electrode assembly in the ionization chamber type X-ray detector shown in FIG. 1. As shown in FIG. 2, the two electrode supporting plates 1 of insulating material, a surface of each plate being provided with grooves 5 arranged with a predetermined interval are disposed with a fixed spacing therebetween. The anode and cathode electrodes 2 and 3 are alternately inserted in these grooves. The ends of each electrode are cemented by binding agent 4 in the grooves 5. The space between a pair of anode and cathode electrodes 2 and 3 defines one detector element of the ionization chamber type X-ray detector.
With this type of X-ray detector, the electrode-electrode distance d must be decreased to increase the density of the detector elements. This necessitates the reduction in the creepage distance along the surface of the insulator. Accordingly, it is difficult to maintain the insulating resistance between the anode 2 and the cathode 3 at a large value. Namely, the dark current from the anode 2 flows via the surface of the insulator into the cathode 3 so that it is impossible to derive a signal current stably from the cathode 3.
FIG. 3 shows an example of the ionization chamber type X-ray detector which can solve the above problem. An electrode supporting plate 1 comprises an insulating member 7 (e.g. of glass) and a conductive member 6 disposed on the insulating member 7 with its contact surface 11 rigidly bound to the member 7, a plurality of grooves 5 being cut at a predetermined interval in the insulating member 7 and the bottom of each groove reaching the contact surface of the conductive member 6. Two such electrode supporting plates 1 (in FIG. 3 only one of them is shown for convenience' sake) are arranged at a distance from each other. The ends of the anode and cathode electrodes 2 and 3 are alternately inserted in the grooves 5 and the plates 1 serve to support these electrodes 2 and 3. As shown in FIG. 3, only one side surface of each of these electrodes is bound to the side surface of a groove 5 with adhesive agent 8 which may be a thermoplastic resin.
With the above-described structure in which a gap is left between the other side surface of the electrode and the side wall of the groove 5, the dark current flowing out of the anode 3 along the surface of the insulating member 7 flows along a path indicated by an arrow 9 into the conductive member 6. Therefore, if the conductive member 6 is grounded, the dark current which might otherwise flow from the anode 2 into the cathode 3, can be eliminated so that the output signal can be detected stably.
The X-ray detector having such a structure as described above has proved, according to the present inventors' experiments, to have the following properties.
Namely, the surface condition of the insulating material largely affects the dark current flowing along the surface of the insulating member. For instance, if the surface is locally contaminated due to an incomplete cleaning of the surface after groove cutting or due to a worker's carelessness during assembling process, the surface resistance of the stained surface portion will increase to increase the dark current flowing therethrough. This causes the uneven distribution of potentials developed over the surface of the insulating member of the electrode supporting plate 1. Accordingly, this uneven distribution of potentials affects electrons flowing into the cathodes 3 so that small undesirable variations appear in the outputs from the respective cathodes 3.
Even, if all or a part of the surface of the insulating member is completely clean and if there is no dark current in the region, photoelectrons generated in the detector may be accumulated on the insulating member surface and therefore cause the surface to be electrified. Moreover, since this phenomenon of electrification fluctuates with time, the distribution of potentials over the insulating member surface is disordered again, which also affects the electrons flowing into the cathodes 3 so that the outputs of the cathodes 3 would contain small fluctuations.